1. Field of the Invention
This invention relates to fuel resistant polymeric elastomers and to articles formed therefrom useful as encapsulants, sealants potting compounds, end seals, coatings and dams for electrical and telecommunication devices and as fuel resistant sealants for fuel containers in the transportation industry.
2. Description of the Related Art
Elastomers and sealants containing polyurethanes are well known in the art. It is also known that the polyols, used to produce such polyurethanes, provide elasticity and strongly influence the chemical and fluid resistance of the system.
For example, polypropylene ether polyols are typically used in applications where resistance to hydrolysis and low temperature flexibility is important, however they readily absorb hydrocarbon fluids. Polyester polyols are used when hydrocarbon fuel resistance is important, but they are highly susceptible to hydrolytic attack. Protective ingredients must be added to polyester based polyurethanes to decrease their susceptibility to attack by water if they are to be used in any application which requires even a moderate hydrolytic stability.
Evidence therefore suggests limitations in performance of polyurethane elastomers where fuel resistance combined with low temperature flexibility is sought using only one type of polyol. With the current state of the art, it appears that polyurethane elastomers that inherently exhibit high polarity and fuel resistance also become undesirably inflexible at low temperatures.
For example, certain polyester polyols, in combination with cycloaliphatic polyisocyanates, an amine coupling agent, and a "cement component" containing an epoxy, can be used to prepare hydrocarbon-resistant polyurethanes as disclosed in U.S. Pat. Nos. 4,668,535 and 4,565,729. The isocyanate index was taught to range from 80 to 120, desirably from 90 to 95. It is specifically stated that unsuitable fuel resistance results from the use of too little isocyanate, i.e., an isocyanate index below 80.
U.S. Pat. Nos. 4,487,913 and 4,247,678 disclose the use of methylene bis(4-isocyanato cyclohexane) and mixed polyesters to achieve fuel resistance. Three component mixtures comprising polyester isocyanate prepolymers, amine or ketimine curing agents, and a "cement component" are used to line fuel tanks in U.S. Pat. Nos. 4,554,300; 4,554,299 and 4,496,707. An epoxy compound was included in the "cement" component. Again, it is taught that compositions having an isocyanate index below 80 exhibit unsuitable fuel resistance and/or hydrolysis resistance.
Fuel resistant translucent pasty sealants are obtained in U.S. Pat. No. 3,158,586 from polyester and polyether isocyanate prepolymers, aromatic amine crosslinkers, and liquid diglycidyl ethers. The active hydrogen materials are described as being present in about equal amounts to the available isocyanate groups thereby requiring an isocyanate index of about 100.
U.S. Pat. No. 5,001,167 discloses a polyurethane polymer based on a polyether polyol containing at least 50 weight percent oxyethylene units which is suitable for hydrocarbon environments. However, it is taught that the polymer is preferably used in environments with little water exposure. Water causes swelling of the polymer due to the high oxyethylene content in the polyol segment. It is further taught that polymer dispersions in polyols should be avoided due to the undesirable hardness of elastomers including such dispersions.
Polyurethane foams and elastomers are derived from linear and branched polyols and polyisocyanates at an isocyanate index of from about 65 to about 85 as taught in U.S. Pat. No. 4,722,946. It is specifically stated that isocyanate indices below 65 do not give a product with useful consistency. It is also taught in this reference that elastomers disclosed in U.S. Pat. No. 4,476,258 having an isocyanate index below 65 could not be duplicated, but attempts instead provided liquid materials, which are not useful for the purpose intended. U.S. Pat. No. 4,476,258 had discussed elastomers from urethane forming components in less than stoichiometric amounts.
It would be extremely desirable to prepare elastomers exhibiting resistance to hydrocarbons from relatively inexpensive polypropylene ether based polyols. The use of propylene ether polyols, in polyurethane formation, would yield elastomers combining resistance to hydrolysis with good low-temperature flexibility. This contrasts with polyurethanes based on highly polar, fuel resistant polyester polyols.
It has now been discovered that fuel resistant, flexible, elastomer compositions result when polyurethane formation, between polypropylene ether polyols and multifunctional isocyanates, occurs in a fluid medium that also contains an excess of polypropylene ether polyol, a dispersed polymer comprising a condensation or addition polymer and free epoxy resin. With the three components of polypropylene ether polyol, dispersed polymer and epoxy resin, present with the polyurethane, there is a surprising improvement in the fuel resistance of the resulting elastomer composition. Although the basis for improvement is not fully resolved, there appears to be a synergy amongst the components of the elastomer compositions of the invention such that omission or significant reduction of any one component severely impairs effectiveness for fuel resistance. Compositions thus impaired show weight gains of 30% to 50% or more after immersion of the elastomer in a hydrocarbon fluid such as kerosene. Elastomer compositions of the invention otherwise show minimal weight gain when soaked in kerosene. Typically, there is less than 10% weight gain for these compositions. This advantage is maintained over a significant range of viscoelastic characteristics. For example, elastomer compositions comprising polypropylene ether polyol, dispersed polymer, epoxy resin and polyurethane were prepared with properties ranging from firm elastomers to jelly-like sealants.
Sealants of the present invention are particularly useful for automotive, electrical and telecommunications cable splice closures and related applications. They provide a protective, fuel and water resistant barrier, e.g. for sealing splice cases or closures for wires or wire pairs or fiber optic fibers or other outside telecommunication apparatus from damage caused by environmental exposure. Historically, such applications employed sealants based on butyl rubber that do not perform well in the hydrocarbon contaminated environments.